The invention has as its object a sensitive display device comprising a scanned screen.
A scanned cathode-ray tube of the television type is by far the most widely applied output interface between a computer and a user. The corresponding input interface is almost always a keyboard of the "typewriter" kind for general applications, merely comprising a few push-buttons for some particular applications. This input interface frequently represents a bottleneck, firstly because it demands constant attention from an untrained operator, which distracts them from the screen, secondly because it has but little flexibility and does not permit an easy reconfiguration as a function of the application, and finally because its cost and its lack of reliability tend to cause a considerable lessening of sales expectations for the greater number of its applications.
In order to eliminate these shortcomings, a number of devices have been proposed which enable the operator to point at a particular spot on the cathode-ray screen in reply to a question or to indicate, for example, a selection from a menu.
The best known is the photostylus or "light-pen" where a probe, held by the operator, containing an optical system and a photodtector and connected via the appropriate electronic linkages to the computer, indicates to the latter the position on the screen at which it is aimed by means of the relative phase of a peak corresponding to the passage of the light spot it detects with respect to the scanning sequence. This is the most efficient system since it is exceedingly simple and permits an excellent resolution (for example 10.sup.4 dots). It has the disadvantage however of requiring an object to be handled, and brings a definite degree of weakness to the system because of the flexible electrical connection joining it to the terminal.
Two different categories of methods have been proposed to eliminate this disadvantage and which enable an operator to point directly to a position on the screen with a finger:
In the first category, it is the contact or pressure of the finger on a transparent plate interposed between the screen and the observer which is detected, either by conduction (plastics material sheets which bear electrodes in cross-patterned matrix form, or a glass panel coated with a layer of conductive oxide and exploiting a Wheatstone bridge effect), or by variation of the acoustic impedance displayed by the point touched.
In the second category, it is the position of the finger which is detected because it interrupts at least two intersecting light beams, of which one forms part of a vertical grid (for example row of photoemitters situated at the bottom of the screen and row of photodetectors situated at the top of the screen) and the other forms part of a horizontal grid (emitter respectively at the right and left of the screen, for example).
What these systems have in common are a comparatively poor resolution, a comparatively low reliability especially for an operator who is not conversant with the process employed, and an electronic system comprising a great number of components.
The device of the invention enables these disadvantages to be overcome by making use of a plate of a transparent material positioned close to the cathode-ray screen, a photodetector being installed at the edge of this plate.